Prevent racy behavior breaking OOBE welcome dialog

It seems that the dialog's show() can be invoked prior to attached(),
resulting in a null object being indexed. This adds a null
check to prevent the race from completely breaking login flow.

Bug: 1028852
Change-Id: I9bfae2e420b1a694a77479f49aa4633089689a54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948185
Auto-Submit: Ken Rockot <rockot@google.com>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#720873}

chrome/browser/resources/chromeos/login/oobe_welcome_dialog.js

@@ -387,7 +387,10 @@
      */
     show: function() {
       this.focus();
-      this.welcomeVideoController_.play();
+
+      // TODO(https://crbug.com/1028852): Investigate why this can be null.
+      if (this.welcomeVideoController_)
+        this.welcomeVideoController_.play();
     },
 
     /**